In directional valves, it has been common to utilize a closed loop position control system wherein the directional valve includes a hydraulic piston that drives the load. The hydraulic piston is moved by a force motor which receives an electrical signal and applies hydraulic fluid to move the piston which, in turn, controls the flow to an actuator that moves the load. A feedback is provided to return the force motor to its original or null position thereby stopping the spool movement at the desired point determined by the size of the initial electrical command signal to the motor. One commonly utilized type of electrohydraulic servo valve is known as the flapper type servo valve such as shown in U.S. Pat. Nos. 3,023,782 and 3,228,423 wherein the force motor comprises a torque motor that moves a flapper that, in turn, controls the flow between opposed nozzles to move the spool. Feedback is achieved by mechanical linkage between the flapper and the spool.
It has also been known that control of the hysteresis inherent in the electrical motor of electro-hydraulic servo valves can be achieved by use of a feedback transducer such as a potentiometer, linear variable differential transformer or the like such as shown in U.S. Pat. Nos. 2,964,059, 3,464,318 and 3,646,762. However, in the case of failure or malfunction of the transducers, the entire electro-hydraulic valve system becomes inoperable.
Accordingly, among the objectives of the present invention are to provide an electro-hydraulic servo valve system which utilizes an electrical sensor to provide feedback signals wherein the system includes another feedback system and control means are provided so that when the electrical sensor fails or malfunctions, the feedback system including the sensor is disabled and another feedback system becomes operable.
In a preferred form, the electro-hydraulic servo valve system comprises a two-stage spool type valve including a first stage comprising an electrical force motor and a second stage including a spool for controlling flow to an actuator. The force motor is operable upon receipt of a command electrical signal to move the spool. The system includes a first feedback operable to cause the force motor to stop the movement of the spool and a second feedback operable to stop the movement of the spool at a predetermined position. The second feedback means has a greater gain than said first feedback so that the second feedback normally dominates in the system. The second feedback comprises a pair of identical electrical sensors connected in mechanical parallel, and means for comparing the electrical signals from the sensors and operable when the signals deviate from one another by a predetermined amount to disable the second feedback so that the first feedback will function permitting the electro-hydraulic servo valve system to operate without the second feedback.